An in vivo mutagenesis assay system will be developed by incorporating in to a transgenic mouse a well defined mutagenesis target, the lacI gene, which has been used extensively in bacterial mutagenesis assay systems. Because it is a forward mutation assay, the nature of mutation at a wide range of sites can be investigated, including 65 sites with the potential to mutate to nonsense codons. The target gene will be incorporated into a lambda phage based shuttle vector and inserted into the mouse genome. The shuttle vector is recovered from genomic DNA preparations by exposing genomic DNA to lambda packaging extracts and using the packaged phage DNA to infect E. coli. The mutation frequency is determined by a color assay on indicator plates. Once mutations in the lacI gene are identified, the gene DNA alteration can be determined by sequencing the target gene following "automatic excision" of the gene using the lambda ZAP excision vector. This system will be able to detect, quantitate and determine the precise nature of mutations in the target gene in cells from any somatic or germ cell tissue removed from the transgenic mouse.